1. Field of the Invention
The present invention relates to processes for forming metal and/or ceramic parts and in particular to molding processes for forming metal and/or ceramic parts.
2. Brief Description of the Prior Art
Porous metals are of interest as structural materials where high specific stiffness, defined as the ratio of stiffness to density, is desired, such as for metal parts for a variety of applications.
Currently, a number of methods exist for producing porous metal structures.
One is by constructing a honeycomb or similar structure by bonding, brazing, welding or diffusion bonding individual components forming the structure.
Another way of producing porous metal structures is by introducing gas into metallic melts. For example, aluminum alloy melts can be exposed to hydrogen, which dissolves in the molten metal. The dissolved gas is released upon solidification of the melt resulting porosity. The porosity generated by this method is not controlled and varies in uniformity and size. For this reason, this technique is not commercially useful.
Yet another technique of producing porous metal structures relies on soaking a polymer sponge with a slurry consisting of metal powder and a polymer binder. The soaked sponge is subsequently dried and fired to burn off the polymer sponge skeleton, leaving behind a metal skeleton that is subsequently sintered to a porous metal part. The shape of the porous metal structure is dictated by the shape of the sponge. Structures with highly interconnected porosity can be formed by this technique. The parts produced by this technique are used as filters and catalyst supports. The pore size of the metal parts produced using this technique is generally large. It is difficult to produce parts that have pore sizes smaller than 1 mm. Furthermore, this technique cannot be used to produce complex parts or structures requiring closed porosity or good surface finish.
Foaming agents of various types have been used to produce porous metal structures. The foaming agents are incorporated into the solid metal. In a version of this process aluminum alloy metal powder is mixed with titanium hydride and the mixture is formed into shapes, such as sheets and rods, the shapes thus produced are then heated above the melting point of the aluminum alloy and the foaming agent decomposes releasing hydrogen that foams the metal. This foamed liquid alloy must be quickly cooled to preserve the porous structure. However, this process is difficult to control because of small a processing window. Since metals have very low viscosity compared to polymers, the growth of the gas bubbles can proceed very rapidly, resulting in large pores. This process generally results in pores that are larger than one millimeter in size. The pore size and distribution are generally not very uniform. This process is being commercialized for simple shapes such as sheets and rods. Complex shapes are more difficult to produce by this method.
There are other processes that are based on the same principle wherein the foaming agent is part of the metallic system. For example, when iron ore is reduced using hydrogen, a porous structure results because the product of reaction causes the structure to form pores. Such metals are called sponge metals. The pores are generally interconnected and large. This process is hard to control and is not used in commercial production of the structural parts. Similar structures are also produced in a process commonly called self-propagating syntheses. An example of this process involves burning titanium metal powder in an atmosphere of nitrogen gas. The titanium metal powder is placed in a container and is ignited at a predetermined temperature. The chemical reaction leading to titanium nitride generates enough energy to heat adjoining titanium powder to continue this reaction. Porous titanium nitride is generally produced in such a reaction.
Porous metal structures can also be produced when a sintering operation in a powder metal fabrication process in not taken to completion. For example if a pressed powder metal part consisting of more than 50 volume percent porosity is only lightly sintered to form bond between particles, a porous structure containing interconnected porosity results. These structures are commercially used as filters for fluids and in self-lubricating bearings. The primary disadvantage of this process is the interconnectedness of the pores and the large pore size. When attempts are made to produce closed porosity using this technique, generally low porosity results.
There is a need for a method of producing porous metal parts of well-defined shape with high proportion of small, closed porosity and good surface finish.
There are a variety of processes known for producing microcellular foams using synthetic organic polymeric materials. One such process employing an injection-molding machine is disclosed in International Patent Application WO 98/31521 and assigned to Trexel, Inc. In the Trexel process a molten polymer is mixed with a supercritical fluid, generally carbon dioxide or nitrogen. The supercritical fluid is intimately mixed with the polymer during the process. Gas bubbles are nucleated by rapid decompression of the supercritical fluid/polymer mixture. The process is controllable and can produce polymer parts containing varying proportion of porosity of various size ranges. The process is well suited for producing parts that have from 10 to greater than 90 percent porosity, with pore size ranging between 10 to 100 microns.
Processes such as extrusion and injection molding can be modified to produce parts using this technology. A number of polymers, including polyethylene, polystyrene, and polypropylene can be processed using this process.
Metal injection molding (“MIM”) is a process that is extensively used for producing net shaped, intricate metal parts. This process is disclosed, for example, in U.S. Pat. No. 4,734,237. In the MIM process, fine metal powder is mixed with a binder phase to produce feedstock for an injection molding operation carried out at a later stage. The binder phase essentially consists of a component that can hold the metal particles together after the molding process and is easily removed via chemical leaching or heat before the sintering operation. A number of other chemicals are added to modify the properties of the slurry to make it more amenable to molding. These include dispersants, wetting agents, etc. The process of removing binder by chemical leaching and/or thermal reaction from a metal injection molded shape is called debinding or debindering. Once the parts are debindered they are sintered under appropriate conditions to produce metal parts. This process has been used to produce metal parts that have low porosity.
Two types of binders have been used in the MIM feedstocks: thermoset and thermoplastic. The thermoplastic binders are by far the most popular. There are a number of proprietary and non-proprietary binder systems in use in industry. Some of the common binders are based on polyethylene, polystyrene or polypropylene, polysaccharides, et al.